Enhancing Solid Oxide Fuel Electrode Performance via Structural and Exsolution Approaches for Operation in Multiple Fuel Environments

Geisendorfer, Nicholas R

doi:https://doi.org/10.21985/n2-d5m6-hq45

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Enhancing Solid Oxide Fuel Electrode Performance via Structural and Exsolution Approaches for Operation in Multiple Fuel Environments

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In the face of a changing climate caused by anthropomorphic release of carbon dioxide and other greenhouse gases, major governments have committed to the reduction of CO2 and other emissions over time, requiring increased reliance on forms of carbon-free renewable energy. The inherent intermittency of renewable electricity sources creates a need to store energy during periods of peak electrical supply, and to draw upon stored energy during periods of inadequate supply. Solid oxide cells, operating reversibly both as fuel cells for power generation and electrolysis cells for fuel gas production, are a promising energy conversion and storage technology exhibiting high round-trip efficiency, modular scalability, discharge times at grid-relevant power loads, and fuel flexibility, capable of operating on hydrogen, carbon monoxide, and hydrocarbon fuel mixtures. This dissertation presents multiple approaches to improving performance of solid oxide cells. In one approach, 3D-printing methodologies are developed which allow the extrusion of a broad range of materials based on several new binder systems. One of these proves effective at enabling 3D-printed components to be incorporated into solid oxide cells as advanced fuel electrode support layers. Fuel electrode-supported solid oxide cells have the principal advantage of enabling the electrolyte to be very thin, hence dramatically lowering cell ohmic resistance compared to electrolyte-supported solid oxide cells. However, because fuel gas must diffuse through this thick, porous microstructure, the superior performance of fuel electrode-supported cells is often somewhat offset by mass transport limitations. A new 3D-printable ink formulation is developed which enables integration of 3D-printed parts directly with tape-cast layers. A solid oxide cell containing a 3D-printed electrode support is fabricated and tested, and compared to a similar cell possessing a traditional support layer. The cell with 3D-printed support layer exhibits no evidence of concentration polarization at high current density in fuel cell mode, and analysis of cell behavior reveals the absence of any limitations associated with gas diffusion, unlike the traditional cell under the same conditions. Overall, this methodology presents a pathway for improving solid oxide cell performance via primarily structural, rather than chemical, means and can in principle be extended to the full range of electrode support materials presently utilized in the field. The development of doped mixed-ionic-electronic-conducting (MIEC) oxide electrodes which exhibit the exsolution of reducible cations, forming metallic nanoparticles embedded on the surface has received significant attention. These exsolved nanoparticles mitigate some of the shortcomings of MIEC electrodes, particularly losses associated with the lack of a metallic electrocatalyst. In another approach to improve solid oxide cell performance, we present an electrochemical study of three recently developed titanate fuel electrodes, SrTi0.3Fe0.7O3-δ (STF), a single-phase electrode, and Sr0.95Ti0.3Fe0.63Ni0.07O3-δ (STFN) and Sr0.95Ti0.3Fe0.63Ru0.07 O3-δ (STFR), both of which exhibit exsolution in fuel atmospheres. Electrodes are tested in both H2/H2O and CO/CO2 fuel gas mixtures across a range of temperatures. The results indicate that while both STFN and STFR significantly improve performance compared to STF in H2/H2O gas mixtures, only STFR offers significant improvement when operating in CO/CO2. Titanate electrodes are also compared against Ni-YSZ fuel electrodes, where they are broadly comparable in H2/H2O at 800°C, and superior in CO/CO2 operation at both 800°C and 650°C.

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